One managed command stored on a mutation topic. payload is the canonical
typed request encoded with the normal wire framing for command_code, so
append and fold share one request schema instead of maintaining a second
mutation representation.
Position of one applied mutation on the KV mutation topic: the fold
coordinate a barriered read waits for. A lease grant or renewal returns the
position at which the answering fold applied it, and a
crate::kv::KvGet carrying it as min_position must not be answered by
a fold that has not applied that position yet. Positions compare only
within one topic_generation: a generation mismatch is fail-closed
(crate::kv::KvError::Stale), never an implicit reset.